In the electronics arts, semiconductor devices are provided in packages that protect and provide external connections to the integrated circuits. The need for integration and advanced functionality of the devices has resulted in multiple integrated circuits, sometimes called chips or dies, being provided in a single package. The package can be formed in various ways and of various materials, including molded packages formed from thermosetting or thermally cured material, e.g., “glob top” or epoxy packages, pre-formed plastic or ceramic or metallic bodies and the like. The materials used protect the small and brittle semiconductor integrated circuits (or “dies”) from physical and some degree of moisture damage, and provide protections for the conductive leads or wires used to couple the external terminals, typically metallic or other conductive contacts, to the conductive bond pads on the integrated circuits, which are the external electrical connections of the integrated circuits.
Often, leadframes are used in the semiconductor packaging art, to provide mechanical support and to make electrical connections between the integrated circuits and the exterior electrical contacts or leads of the packaged device. Leadframes consist of conductive material, often copper or an alloy, or an iron nickel alloy such as Alloy 42, which are often coated to increase conductivity and solderability with materials such as gold, ruthenium, palladium, and the like, and additional coatings or alloys of nickel, copper, or other materials may be used to improve solderability and the manufacturability of connections. Plastic coatings over conductive material may be used to form a leadframe. Leads may be soldered or plated for soldering prior to assembly, or after assembly of the completed package. The leadframes are usually provided in an integrated strip form, and may be etched or stamped out, the leadframe strips are several leadframes connected in a strip form for ease of assembly and manufacture, and are then separated at a later stage of manufacture.
The leadframe typically provides a plurality of leads, often finger-shaped, although other shapes are used, which extend from outside an area that will be the exterior boundary of the desired finished package to an interior area arranged to receive an integrated circuit. In the prior art, it is known to use an arrangement where one area of the leadframe provides a central support, called a “die pad”, for receiving the rectangular or square semiconductor die, and the lead fingers extend on one or more sides of the die pad to an area proximate to the outside edges of the integrated circuit die. The leadframe fingers are positioned extending away from the die and through the planned exterior boundary of the encapsulated package. In other arrangements of the prior art, the lead fingers may extend over the die (lead on chip or “LOC” type leadframes) or under the die (lead under chip or “LUC” type leadframes) with the leads providing mechanical support, as well as electrical paths.
Insulator adhesive in the form of coatings or tapes can be used to secure the die to the leads, or to stabilize the leads by securing the leads together and maintaining their position during the assembly process. The die may be adhered to a die pad using a die attach adhesive, which may be conductive or insulating material and may be a resin or thermosetting material.
Regardless of the type of leadframe used, it is necessary to provide coupling mechanisms to electrically couple the integrated circuit to the leadframe. Typically, bond wires are used. These miniature wires are applied to the semiconductor device by a wire bonding process; the wire is typically dispensed as it is applied through a capillary. The wire bonding process uses heat and pressure and sometimes other energies, such as ultrasonic energy to form a bond by attaching a wire to the integrated circuit bond pad, and then extends the bond wire above and away from the integrated circuit and to an area above the end of a lead finger of the leadframe, then the capillary again uses heat and pressure to form a second connection of the bond wire to the leadframe. Alternatively, the bond wire could be formed in a vice versa direction, attaching first to the leadframe finger and extending up to and above the integrated circuit and attaching to a bond pad. The cut wire is often heated to form a ball on the end of the bond wire, which is then used for the next bond to the integrated circuit die (“ball” bonding), the end of the bond wire which is attached to the lead frame without a ball may be called the “stitch”. If required, multiple bond wires can extend from different pads of the integrated circuit to a single lead of the leadframe, for example, power or ground connections for the integrated circuit may be made in this manner. The bond wire may be a gold or other known conductor material that is malleable and flexible enough to allow for this type of handling and which is useful for the ball and stitch bonding steps without unwanted breakage. The wire bonding process may be highly automated, and is typically performed at a very high rate of speed.
In the assembly process, after the integrated circuit die is wire bonded to the leadframe, the leadframe and die may be placed in molding equipment, for example in a transfer molding machine, where liquid or molten mold compound material is dispensed to encapsulate the leadframe and integrated circuit together, so as to provide mechanical protection and some degree of moisture resistance to the die as described above. Other alternatives include using injection molding, epoxies and resins, such as “glob top” materials and other known materials for integrated circuit encapsulation may be used. In lieu of molding, the leadframe and die assembly may instead be installed into a ceramic, metallic or plastic body which may then be subsequently sealed by encapsulant, using lids and adhesives, or otherwise. The external ends of the leads of the leadframe may themselves form the external contacts of the packaged device, such as in a DIP, quad flat pack, SOP, or other leaded package, or additional connectivity technologies may be used, such as in a ball grid array (“BGA”) or pin grid array (“PGA”) package and the like. Leadframes may be used in combination with other interconnection interposer technologies, such as printed circuit boards, flexible circuits based on film based materials, commercially available films used in semiconductor manufacture, such as Kapton, Upilex, Mylar and the like, or ceramic substrate materials may be used. To make complex substrate arrangements possible in the prior art, multiple layer interposers are often used with metal layers formed coupling external connectors to the integrated circuits. For example, a terminal on the bottom surface may be coupled to the leadframe or wire bonding land terminals on the upper surface of the interposer through multiple layers and vias in the substrate or interposers. These interposers or substrates are typically laminated structures with insulator layers formed over various conductive layers. Once the assembly is completed, these laminates may be overmolded to provide a hermetically sealed packaged device or the assembly may be placed in a body that is sealed.
As the need for increased integration in packaged devices continues, it is also known in the art to provide a MCM, or multiple chip module, in which more than one integrated circuit die is provided within a packaged device. For example, a memory device and a controller may be packaged together to form such a module. A processor and memory may form a module as well. These devices may alternatively be identical devices, for example to form a large memory integrated circuit such as commodity DRAM or nonvolatile memory device, multiple identical dies may be placed in one package, with the common terminals of such devices coupled together in parallel to external contacts of the package.
In order for the multiple integrated circuits to be coupled together in a system configuration, various techniques are used. Flex circuits may be formed, which have metallization patterns provided on one or both sides of a flexible substrate, these then act as interconnect levels for connecting the two integrated circuits together. Laminates, such as FR-4 or BT resin cards, can be formed with multiple metal layers and interlevel via technology, these laminate interposers again act as a miniature circuit board for connecting the integrated circuits together and provide traces for external connectivity, such as terminals.
When identical devices are to be coupled together to increase integration, for example as in the case of DRAM devices, die stacking may be used. Bond wires may be extended from the leads of a leadframe to several dies, for example the address leads of a DRAM package may be wired to several DRAM integrated circuits that are stacked. Stacking of dies may include spacers between the dies to enable the wire bonding equipment to access the die pads of the individual stacked integrated dies.
When identical devices are to be coupled together to increase integration, for example as in the case of DRAM devices, die stacking may be employed. Various methods can be used such as providing multiple dies in a “face up” arrangement, and wire bonding may be formed to couple each die to a common leadframe using bond wires to couple them in parallel. It is known to place the dies in a back-to-back relation on leadframe, however, in order to maintain a common bond pad footprint, when a back-to-back relation is used, a “mirror die” is often required, so that the terminals on one side of the die facing upwards will be located in the same position and order as on the corresponding die facing downwards. The need for a “mirror die” greatly increases the complexity of manufacture, inventory control and costs, and requires that each packaged device contain two different dies, having the same function. Alternatively, an interposer or laminate circuit could be used to enable the back-to-back positioning of two identical functional die, this laminate interposer also adds cost and complexity to the finished device.
A particular packaged device type of increasing commercial importance in recent years is the removable non-volatile storage card, allowing transfer of data between a variety of electronic devices. This nonvolatile memory or storage card is available in a variety of formats, including Compact FLASH, Secure Digital or SD, mini-SD, Memory Stick, USB drives, Multimedia Card or MMC, and other formats. In order to provide a robust, reliable and stable data storage format, nonvolatile EEPROM or FLASH memory devices are provided along with an intelligent controller in a single packaged device. The intelligent controller provides data error correction and detection, test, cache and redundancy support function so that even though some storage locations within the nonvolatile memory device are expected to fail and do fail during the useful life of the product, the user data is stored and retrieved correctly and the user or system is unaware that some locations within the memory array are no longer used; the intelligent controller replaces these locations with redundant memory locations and maintains a map of the available locations which is used to maintain the proper storage and consistency of the data. To the user system, the device looks like a large memory array, the controller and the automatic error correction features and redundancy support provide user transparent automatic memory control operations which does not affect the use of the device. These removable storage cards are used and will continue to be used in many applications where data is stored, particularly for cell phones, digital cameras, digital media storage, such as MP3 music and video for music players, video players, electronic games, personal data assistant or PDA devices, for medical history storage, smart cards, credit cards, and the like.
FIG. 1 depicts the exterior surfaces of a typical removable storage card package. This card may be of the type, for example, as described in U.S. Pat. No. 6,410,355 to Wallace, the inventor of the present application, which is incorporated herein by reference. In FIG. 1a, a contact side of the card, for example a secure digital or SD format card, is depicted with conductive terminals 101 arranged for contacting the integrated circuits within package 100. FIG. 1b depicts the opposing side of the package 100, which has no electrical contacts, but typically carries a label with information, brand name, size of media, and the like for the user's visual inspection and reference. The number of terminals, and the type of connections used, varies with the format, for example for secure digital or SD, the terminals shown in FIG. 1a are typical, and only a few external terminals are used. For compact flash or “CF” cards often used with digital cameras the number of terminals is greater, and the terminals are female receptacles located on one end of the side of the package. The camera or card reader has a socket for receiving the same end of the CF package with male terminals or pins within the socket that enter the corresponding female receptacle when the compact flash card is inserted into the socket, thereby completing the connection. Other connections may be used, for example a USB port may be used as the connection.
The prior art packages for removable storage card devices typically include a complex interposer or substrate in the form of a multiple layer laminate printed circuit board or “PC board”, which provides physical support and device-to-device connectivity for the controller integrated circuit and the memory device or devices. The board, which may be of BT resin, FR4, or fiberglass or similar, usually is a laminate structure that incorporates metal layers which are patterned to form conductor traces, vias coupling the various layers for making electrical connections, and lands for wire bonds to couple traces on the surface of the board to integrated circuit dies or other components, packaged or bare die components, which are mounted to the board. Multiple memory devices may be provided, for example side by side or as a stack, or a single memory device may be used, but in any event the storage card as packaged in the prior art is a complex packaged device with at least two devices packaged in it and coupled together. FIG. 2 depicts a typical arrangement in a cross sectional view. In FIG. 2, a storage card 200 of the prior art is illustrated having integrated circuit dies 204 and 205 mounted on the same surface of laminated substrate 208. Bond wires 203 connect bond pads on the active surface, or face, of the integrated circuit dies to conductive areas, or lands 206 on the upper surface of the substrate. Two such bond wires are shown coupling two die pads of the integrated circuits electrically by being connected at land 206, the integrated circuits are thus coupled electrically and may be, for example, a memory and a controller integrated circuit. Die attach material 209 is used to secure the dies 204, 205 to substrate 208. After a conventional semiconductor package assembly process that forms the bond wires 203 and attaches them to the integrated circuit dies 204 and 205 and the lands 206 on the substrate, the bond wires and integrated circuit dies are encapsulated in encapsulant 211, which may be thermosetting or room temperature mold compounds or other encapsulating material. The package is completed with shell 201, which may be plastic, covering the substrate and molded materials. In another approach, U.S. Pat. No. 6,639,309 to Wallace, the inventor of the present invention, also incorporated herein by reference, depicts a removable storage card incorporating a memory device and a controller device on opposing surfaces of a multilayer PC board material, with wire bonding connections, and an overmolding encapsulation.
Other approaches to packaging semiconductor integrated circuits may incorporate multiple leadframes or multiple level leadframes that are coupled together. For example, U.S. Pat. No. 5,147,815 to Casto, which is incorporated herein by reference, depicts two integrated circuit dies and two leadframes assembled and provided in a single molded dual inline plastic or “DIP” package. The integrated circuit dies and their respective leadframes are arranged in back-to-back relation and each die is coupled to the respective leadframe by the use of bond wires, or alternatively, the integrated circuits are arranged in a face-to-face relation on opposing sides of an interposer and coupled to their respective leadframe in a flip-chip arrangement, the two integrated circuits are independently coupled to external leads arranged on opposing sides of the packaged device and are not in electrical communication with each other. U.S. Pat. No. 6,603,197 to Yoshida et al., also incorporated herein by reference, provides multiple leadframes coupled to at least two different integrated circuit devices, which are coupled to various leads of the leadframes to form a module, with some common leads being physically and electrically coupled together at the exterior of the package, for example a power lead, so that both integrated circuit devices may receive the signal. Similarly, U.S. Pat. No. 6,316,825 to Park et al., also incorporated herein by reference, provides a stacked package for stacking two identical integrated circuit devices, such as memory devices, in a molded package with two leadframes, which are physically coupled at the exterior of the package, such that each signal coupled to the external leads is physically and electrically coupled to each of the two identical memory devices, which are connected in parallel fashion.
Other arrangements known in the art provide a single integrated circuit coupled to multiple level leadframes, for example, U.S. Pat. No. 5,220,195 to McShane, also incorporated herein by reference, provides a single integrated circuit wire bonded to a multilayer leadframe and includes physical connections between parts of the multiple layer leadframes within the package, and, through-hole vias formed with bond wires extending into the vias to make physical contact to leadframe layers positioned beneath the integrated circuit, thereby enabling the formation of multiple voltage planes within the packaged device.
Although prior art packages for multiple integrated circuits exist, there is a continuing need for multiple die packages that provide reduced costs of production while maintaining the reliability of the package.
A need thus exists for an improved method for packaging multiple integrated circuits, that is simple and reliable, allows for arbitrary connections between various integrated circuit devices, does not require expensive interposers, printed circuit boards, or substrates, and is less costly to manufacture than the existing packages and methods.